Background: Up to 40 percent of injection drug users have a skin or muscle infection (soft tissue infection) at any given time. These infections frequently require hospitalization and operating room drainage and often cause substantial morbidity and mortality. The national economic and health care burden of treating these infections has never been determined. Objectives: The aims of the proposed study are to (1) describe hospitalizations for illicit drug users with soft tissue infections (as a proxy for injection drug use-related soft tissue infections), (2) provide the first national estimates of their inpatient health care utilization and costs, and (3) identify hospital factors associated with increased health care utilization and in-hospital mortality among illicit drug users with soft tissue infections. Methods: This proposed study will use data from the Health Care Cost and Utilization Project Nationwide Inpatient Sample. It contains data on patient and hospital characteristics, ICD-9 discharge and procedure codes, and hospital charges for a 20 percent sample of non-Federal U.S. hospital discharges. Because no unique ICD-9 codes exist for injection drug use or its related infections, hospitalized illicit drug users with soft tissue infections will be identified using combinations of ICD-9 codes that denote the presence of both illicit drug use and a soft tissue infection. This study will describe the hospitalizations for illicit drug users with soft tissue infections, estimate their national health care utilization and charges, and evaluate their trends over time, from 1996-2000. Linear and logistic regression will be used to assess the relationship of hospital types and locations with the proportion of all hospitalizations attributed to illicit drug users with soft tissue infections, and with the length of hospital stay and risk of in-hospital death for illicit drug users with soft tissue infections. Significance: Estimating the costs and health care utilization attributed to hospitalizations for illicit drug users with soft tissue infections will improve the accuracy of current estimates of the costs of substance abuse and improve our understanding of the burden of caring for these infections in the U.S.